[RFC - is this a bug?] wifi: ath10k: Asking for some light on this, please :)

["Gustavo A. R. Silva" <gustavo@xxxxxxxxxxxxxx>]

Hi all,

While working on tranforming one-element array `peer_chan_list` in
`struct wmi_tdls_peer_capabilities` into a flex-array member

7187 struct wmi_tdls_peer_capabilities {
...
7199         struct wmi_channel peer_chan_list[1];
7200 } __packed;

the following line caught my attention:

./drivers/net/wireless/ath/ath10k/wmi.c:
8920         memset(skb->data, 0, sizeof(*cmd));

Notice that before the flex-array transformation, we are zeroing 128
bytes in `skb->data` because `sizeof(*cmd) == 128`, see below:

$ pahole -C wmi_10_4_tdls_peer_update_cmd drivers/net/wireless/ath/ath10k/wmi.o
struct wmi_10_4_tdls_peer_update_cmd {
	__le32                     vdev_id;              /*     0     4 */
	struct wmi_mac_addr        peer_macaddr;         /*     4     8 */
	__le32                     peer_state;           /*    12     4 */
	__le32                     reserved[4];          /*    16    16 */
	struct wmi_tdls_peer_capabilities peer_capab;    /*    32    96 */

	/* size: 128, cachelines: 2, members: 5 */
};

So, after the flex-array transformation (and the necessary adjustments
to a few other lines of code) we would be zeroing 104 bytes in
`skb->data` because `sizeof(*cmd) == 104`, see below:

$ pahole -C wmi_10_4_tdls_peer_update_cmd drivers/net/wireless/ath/ath10k/wmi.o
struct wmi_10_4_tdls_peer_update_cmd {
	__le32                     vdev_id;              /*     0     4 */
	struct wmi_mac_addr        peer_macaddr;         /*     4     8 */
	__le32                     peer_state;           /*    12     4 */
	__le32                     reserved[4];          /*    16    16 */
	struct wmi_tdls_peer_capabilities peer_capab;    /*    32    72 */

	/* size: 104, cachelines: 2, members: 5 */
	/* last cacheline: 40 bytes */
};

This difference arises because the size of the element type for the
`peer_chan_list` array, which is `sizeof(struct wmi_channel) == 24 `

$ pahole -C wmi_channel drivers/net/wireless/ath/ath10k/wmi.o
struct wmi_channel {
	__le32                     mhz;                  /*     0     4 */
	__le32                     band_center_freq1;    /*     4     4 */
	__le32                     band_center_freq2;    /*     8     4 */

[..]
                                               /*    20     4 */

	/* size: 24, cachelines: 1, members: 6 */
	/* last cacheline: 24 bytes */
};

is included in `sizeof(*cmd)` before the transformation.

So, my question is: do we really need to zero out those extra 24 bytes in
`skb->data`? or is it rather a bug in the original code?

Thanks!
--
Gustavo